If i post any question using same email (not registered) on SO, I don't get same userid... If I clear cache / use new browser / post from diff. place. This is me >> https://stackoverflow.com/users/260731/kv Is there anyway I can again use this user? I have not registered.
Two key things:
I clear cache / use new browser / post from diff. place.
When you are not registered, StackOverflow uses a cookie to remember who you are. If you lose that cookie or access StackOverflow from somewhere that can't use that cookie (because you "clear cache / use new browser / post from diff. place") you'll have a different account. You need that cookie to access your account — for all intents and purposes the cookie is the account.
If you want to be able to do things like clear your cache or use your account from more than one machine, the solution is simple: register.